Barlow lenses extend the focal length of the telescope. They are often used to obtain two different levels of magnification with a single eyepiece. If you use a 2x Barlow lens with an eyepiece, the magnification doubles.
In an astronomical telescope, the image is laterally transposed and/or upside down. This is unimportant for astronomical observing, but for terrestrial observing it is important. Erecting lenses provide an upright and true-sided image.